1. Field of the Invention
The present invention relates to a negative pressure type booster which is used to boost a brake force, and more particularly to a negative pressure type booster for use in a vehicle.
2. Description of the Related Art
Conventionally, there has been widely used a negative pressure type booster for reduction of an operation force when braking a vehicle. The conventional negative pressure type booster includes: a constant pressure chamber, which is normally in communication with an intake manifold provided in an engine and into which a negative pressure can be introduced; a variable pressure chamber which is used to select either a first state where the variable pressure chamber is cut off from the constant pressure chamber so that it is in communication with the atmospheres or a second state where the variable pressure chamber is in communication with the constant pressure chamber so that a negative pressure is introduced into the variable pressure chamber; a vacuum valve which is used to establish or cut off communication between the constant pressure chamber and the variable pressure chamber; and an air valve which is used to establish or cut off communication between the variable pressure chamber and the atmosphere. In the conventional negative pressure type booster, if the vacuum valve and the air valve are opened or closed by operating a brake operation member provided in the vehicle, then a pressure difference according to the operation force of a brake operation member can be set between the constant pressure chamber and variable pressure chamber, which makes it possible to generate a brake force which is amplified according to the operation force of the brake operation member.
By the way, in recent years, there has been known a negative pressure type booster including an electrically operated solenoid valve, where even when a brake operation member is not operated by a driver, the operation of the solenoid valve can bring a variable pressure chamber into communication with the atmosphere to thereby produce a large is pressure difference between the variable pressure chamber and a constant pressure chamber, so that a great brake force can be generated. For example, Japanese Patent Unexamined Publication No. Hei 5-24533 discloses a negative pressure type booster of the above-mentioned type.
On the other hand, conventionally, there is also known a brake fluid pressure device including a traction control system (hereinafter referred to as "TRC") in which when a vehicle starts to move, if an excessive drive force is applied onto the wheels of the vehicle to the degree that the drive wheels thereof are caused to slip on the surface of a road, without any braking operation by a driver, a brake force can be applied onto the side of the slipping drive wheels to increase a brake fluid pressure temporarily, thereby causing the slipping wheels to grip the road surface so that the vehicle wheels can be accelerated effectively. In such a brake fluid pressure device, there has been demanded an initial pressure increasing function to allow smoother supply of brake fluid which is used to give a brake force onto the side of the slipping wheels.
When the above-mentioned initial pressure increasing function is carried out in the above conventional negative pressure type booster, the following case can be expected: that is, for example, in the start of motion of the vehicle, if a drive force is given to the vehicle wheels excessively to thereby cause the drive wheels thereof to slip on the road surface, with no braking operation by a driver, the solenoid valve can be operated electrically, the operation of the solenoid valve can bring the variable pressure chamber into communication with the atmosphere to thereby produce a large pressure difference between the variable pressure chamber and the constant pressure chamber and thus generate a brake force, so that the brake fluid can be supplied to the slipping drive wheel side.
However, in the conventional negative pressure type booster, the great brake force is generated because the atmosphere is introduced into the variable pressure chamber to fill it up by means of the operation of the solenoid valve. Therefore, in the TRC or the like, when the conventional negative pressure type booster is operated as an initial pressure increasing function, if the solenoid valve or the like is out of order, then there exists a fear that the great brake force can be generated all the time.